This invention relates to the use of electromagnetic wave energy to superficially alter a substrate, e.g., by ablation and/or photochemical reaction.
Lasers are useful in medical, materials processing, and other applications to cause ablation, i.e., substance removal, within a substrate, e.g., biological tissue or other material. In addition, certain lasers, e.g., ultraviolet (UV) lasers, can be used to cause photochemical alterations, e.g., polymerization, in a substrate, with or without simultaneous ablation.
Ablation of biological tissue by lasers occurs predominantly by the rapid thermal vaporization of tissue water. However, secondary processes may coexist with this thermal vaporization. For example, explosive mechanical removal is caused by short laser pulses when laser energy intensity is high enough to initiate a plasma that produces shock waves and mechanical fracture, e.g., greater than about 10.sup.8 W/cm.sup.2. Additionally, UV pulsed laser ablation can cause concurrent photochemical reactions in tissue. When present, these secondary processes can change the efficiency of pulsed laser ablation.
The ablation depth within tissue or other materials depends upon the depth to which the electromagnetic waves penetrate. For some applications, e.g., treatment of large tumors, deep penetration is required, and appropriate wavelength regions, e.g., red or near infrared, are preferable. For other applications, a well-controlled superficial effect is desired, e.g., ablation of the outer surface of the cornea to correct vision, or of the inner surface of diseased arteries.
Laser energy is typically delivered as a beam or illumination in which the electromagnetic energy propagates directly into the tissue or other substrate. Laser energy may also be delivered in the form of refracted or evanescent fields or waves generated at interfaces between two materials that have different refractive indices. For example, evanescent waves have been used in the technique of attenuated total-reflection (ATR) spectroscopy for absorption spectrophotometry, which is a non-destructive measurement tool.
U.S. Pat. Nos. 5,042,980 (corresponding to European Patent Application No. 400,802) and 5,207,669 describe an optical fiber diffusion tip and the use of evanescent waves to direct laser radiation from an optical fiber, e.g., within an angioplasty balloon outwardly through a portion of the balloon surface to heat the tissue surrounding the balloon. The evanescent waves are generated at an interface between the optical fiber and a cladding material of lower refractive index, which is standard in clad optical fibers.
Ablation has been achieved with laser beams by choosing wavelengths that are strongly absorbed by proteins (far ultraviolet radiation, i.e., 193 nm excimer laser) or water (mid-infrared radiation, i.e., 2.9 .mu.m Q-switched erbium:YAG lasers). In general, the removal of tissue with such strongly-absorbed wavelengths is controlled due to the small "bites" taken with each pulse. This approach also produces the least thermal trauma to surrounding tissue, and creates the smallest-size ablation debris. The relationships between depth of penetration, optical absorption, and pulse duration in determining, e.g., thermal injury, bite size, and efficiency, are well described in the literature. See, e.g., Walsh, J. T., et al., Lasers Surg. Med., 8:108-118 (1988).
However, the same strongly-absorbed wavelength regions are exceedingly difficult to deliver through known optical fiber systems. For example, certain laser angioplasty systems use a special pulse-stretched excimer laser at 308 nm. The pulse-stretching is costly, but is necessary for fiber optic delivery. In addition, the 308 nm wavelength causes thermal damage and is associated with high rates of mutagenesis, but is at present the shortest excimer wavelength that is optical fiber-compatible.
In spite of these limitations, there are examples of the use of strongly-absorbed, short-pulsed lasers for ablation. For example, 193 nm excimer lasers have been used for refractive correction of the eye involving reshaping of the corneal surface. See, e.g., Marshall, et al., U.S. Pat. No. 4,941,093. At present, this is accomplished by sophisticated beam-control systems. Moreover, the required laser is complex in comparison to solid-state lasers.
Laser ablation is also used to remove the stratum corneum, the outermost 8 to 15 .mu.m dead layer of human skin which provides the major chemical diffusion barrier. The use of such ablation to enhance percutaneous transport was developed by Dr. S. Jacques et al. using both 193 nm excimer, and 2940 nm Er:YAG lasers, e.g., as described in U.S. Pat. No. 4,775,361.
Laser ablation has also been used in dental applications, but has been limited, in part, because laser energy at wavelengths capable of adequate fiber transmission propagate deeply into both enamel and dentin layers, causing excessive heating and damage. The preferred lasers for use with conventional delivery devices for dental applications are erbium or hydrogen fluoride (HF) lasers, running at about 3 .mu.m wavelength, or UV excimer lasers, running in pulsed modes. However, these lasers are not generally fiber-compatible. For example, the only laser presently commercialized for dentistry is a normal-mode Nd:YAG laser, whose application is limited to caries removal, and must be used in combination with conventional drilling procedures.
Furthermore, laser ablation has been used to a limited extent in angioplasty. Ideally, ablation would remove significant amounts of plaque, avoid perforating the arterial wall, and leave a smooth cylindrical lumen internal surface with minimal thermal injury. It is well established that 193 nm excimer laser pulses can produce smooth, microscopically-controlled removal of atherosclerotic plaque in vitro. However, 193 nm excimer laser pulses cannot be delivered effectively through optical fibers, and thus, long-pulse 308 nm excimer laser pulses and later 2 .mu.m holmium laser pulses have been used. Unfortunately, these wavelengths penetrate too deeply (50 to 400 .mu.m) into arterial wall tissue to achieve precise ablation, causing large interaction volumes, large vapor cavitation, and tearing, mechanical injury rather than precise, smooth-surface ablation. Known laser angioplasty catheters are forward-shooting devices which are typically passed over a guidewire to avoid perforating the artery, and are used as an adjunct to balloon angioplasty for total occlusions.